In prior art devices, such as in the digital-analog converter (DAC) described in the article by Jean Michel Fournier and Patrice Senn entitled "A 130 MHz 8-b CMOS video DAC for HDTV Application" (IEEE Journal of Solid-State Circuits, volume 26, No. 7, July 1991, pages 1073-1077), one of the methods used for producing a digital-analog converter (DAC) consists of using an array of current sources connected to a load in accordance with an input code of the DAC. Two methods are widely used for obtaining a given resolution of N bits.
1) Use is made of an array of N binary weighted sources controlled directly by the N bits of the input code, as shown in FIG. 1. PA0 2) Use is made of an array of 2.sup.N unit sources controlled by the thermometric conversion of the input code, as shown in FIG. 2
The first solution provides the smallest and fastest configuration, but a matching to N bits of the current sources is necessary in order to ensure monotonicity.
The second solution ensures monotonicity regardless of the matching, but requires a large silicon area which does not correspond to the actual current sources. Even in the case of binary weighted sources, the latter are formed by unit cells in parallel for matching reasons. The increased area is due to the logic necessary for the series selection of each source.
In a typical digital-to-analog converter, as shown in FIG. 3: an N bit input code B[N-1:0] is subdivided into MSB (Most Significant Bits) and LSB (Least Significant Bits), respectively H and V: ##EQU1##
The codes H and V are converted into thermometric codes, respectively HT and VT: EQU HT[2.sup.N -1:0] with HT[j]=1 for j.ltoreq.val(H) and HT[j]=0 for j&gt;val(H) EQU VT[2.sup.N -1:0] with VT[j]=1 for j.ltoreq.val(V) and VT[j]=0 for j&gt;val(V)
The bits of HT and VT codes are respectively horizontally and vertically routed to the source array. A source located in position (h,v) is excited (i.e. its current is supplied to the load) in accordance with the condition: EQU (HT[h] AND VT[v])
Starting with a zero input code on the DAC, only HT[0] and VT[0] are brought to "1". The only selected source in the array is that of the upper left-hand corner. By increasing the code up to the least MSB, all the sources of the first left-hand column are progressively selected When the smallest MSB assumes the value "1", VT[1] is brought to "1". All of the sources of the first column are maintained selected and the sources of the second column are progressively excited (turned on) and so on.
The AND-OR logic necessary for implementing the aforementioned condition is alongside each current source and controls a switch in series with the source, as shown in FIG. 4. In practice, the area occupied by the DAC is dominated by the AND-OR gates.
The object of the present invention is to reduce the area of the thermometric-type common DAC, while maintaining its inherent monotonicity and therefore increasing at the same time the integral linearity of the converter due to the improved matching of the near unit sources.